Interface advance control in secondary recovery program by use of dynamic gradient barrier and by retarding cusp formation

ABSTRACT

The advance of the interface between driving and driven fluids in a secondary recovery operation toward a production well is delayed by the imposition of a gradient barrier of produced hydrocarbon fluids injected into the formation via a control well in line between an injection well and a production well, the recirculation of the formation hydrocarbon fluids providing a dynamic barrier, and by retarding cusp formation via wells controlling the advance of flow gradients to spread the interface.

United States Patent Inventor Donald L. Hoyt Houston, Tex.

Appl. No. 837,782

Filed June 30, 1969 Patented Sept. 14, 1971 Assignee Texaco Inc.

New York, N.Y.

Continuation-impart 01 application Ser. No. 786,568, Dec. 24, 1968.

INTERFACE ADVANCE CONTROL IN SECONDARY RECOVERY PROGRAM BY USE OF DYNAMIC GRADIENT BARRIER AND BY RETARDING CUSP FORMATION ABSTRACT: The advance of the interface between driving and driven fluids in a secondary recovery operation toward a production well is delayed by the imposition of a gradient bar- Claims 16 Drawing Figs rier of produced hydrocarbon fluids injected into the forma- U.S. Cl 166/245, tion via a control well in line between an injection well and a 166/263, 166/268 production well, the recirculation of the formation hydrocar- Int. Cl E21b 43/22 bon fluids providing a dynamic barrier, and by retarding cusp Field of Search 166/245, formation via wells controlling theadvance of flow gradients 263, 268 to spread the interface.

. 6'2 W c 52 I 4 s 6/ 2 l \1 INTERFACE ADVANCE CONTROL IN SECONDARY RECOVERY PROGRAM BY USE OF DYNAMIC GRADIENT BARRIER AND BY RETARDING CUSP FORMATION CROSS-REFERENCES This application is a continuation-in-part application for patent of the copending, commonly assigned application for patent, Ser. No. 786,568, filed Dec. 24, 1968 by Donald L. I-Ioyt for Interface Advance Control in Secondary Recovery Program by Use of Gradient Barrier.

FIELD OF THE INVENTION DESCRIPTION OF THE PRIOR ART In the production of hydrocarbons from permeable underground hydrocarbon-bearing formations, it is customary to drill one or more wells into the hydrocarbon-bearing formation and produced hydrocarbons, such as oil, through designated production wells, either by the natural formation pressure or by pumping the wells. Sooner or later, the flow of hydrocarbons diminishes and/or ceases, even though substantial quantities of hydrocarbons are still present in the underground formations.

Thus, secondary recovery programs are now an essential part of the overall planning for virtually every oil and gas-condensate reservoir in underground hydrocarbon-bearing formations. In general, this involves injecting an extraneous fluid, such as water or gas, into the reservoir zone to drive formation fluids including hydrocarbons toward production wells by the process frequently referred to as flooding. Usually, this flooding is accomplished by injecting through wells drilled in a geometric pattern, the most common pattern being the fivespot.

When the driving fluid from the injection well reaches the production wells of a five-spot pattern, the areal sweep is about 71 percent. By continuing production considerably past breakthrough, it is possible to produce much of the remaining unswept portion. It would be a great economic benefit to be able to achieve a sweep of I percent of the hydrocarbonbearing formation. It would be an even greater benefit to be able to achieve it at breakthrough, so that it would not be necessary to produce large quantities of injected driving fluid.

It is understood that the failure of the driving flood in secondary recovery operations to contact or sweep all the hydrocarbon area is due to the development of a cusp at the interface between the driving and the driven fluids, which advances toward the production well. If other portions of the interface could be made to keep up, or if the cusp formation were delayed, a more complete areal sweep would be possible.

In the commonly assigned U.S. Pat. No. 3,393,735, issued to A. F. Altamira et al. on July 23, [968 for Interface Advance Control in Pattern Floods by Use of Control Wells, there is disclosed how an increased amount of hydrocarbons is produced and recovered from an underground hydrocarbonbearing formation by employing at least three wells, penetrating such a formation, which wells are in line, to produce hydrocarbons from the formation via two of these wells including the middle well, asdisclosed in the commonly assigned U.S. Pat. No. 3,109,487, issued to Donald L. Hoyt on Nov. 5, 1963 for Petroleum Production by Secondary Recovery. In both these cited patents, there is disclosed how a production control well is positionedbetween the injection well and theproduction well and is kept on production after the injected fluid reaches it. In this manner, the cusp is pinned" down at the control well and while the area swept out by the injection fluid before breakthrough at the outer production well is increased, there is an unwanted handling of con siderable quantities of injected fluid at the control well.

Another aspect to increase the sweep is disclosed in the commonly assigned U.S. Pat. No. 3,393,734, issued to D. L. Hoyt et al. on July 23, 1068 and involves the retardation of the development of the cusp toward a production well. The method of achieving more uniform advance is to control the flow gradients so that the interface is spread" out. This can be done either by choosing a particular'geometry of well positions or'by adjusting the relative production rates so that the velocity of advance is not predominantly in one direction. It can be done also by shifting the gradients frequently, in both direction and magnitude, thus preventing any one section of an interface from advancing too far out of line.

SUMMARY OF THE INVENTION It is an overall object of the present invention to provide an improved secondary recovery procedure involving initially three wells, substantially in line, and a fourth well offset from such a line, which wells may be part of a well arrangement for exploiting a hydrocarbon-bearing formation, by changing the function of the wells at strategic times to gain maximum control of the flood front.

Such a four well grouping is arranged with three wells in line substantially and a fourth well offset therefrom so that an end well in line is completed for injection and the remaining three wells are completed for production. Flooding is initiated at the end well by injection of an extraneous driving fluid, such as water or gas, thereinto and proceeds until breakthrough of the flood front occurs at either the closer of the production wells in line or the production well offset therefrom, at which time injection via the end well to maintain flooding is suspended and the production wells are put on a standby basis, e.g., by being shut in. Then, preferably, a small volume or slug of an extraneous fluid is injected into the formation via the produc-' tion well at which breakthrough occured, to drive the flood front away from this well, injection at the end well and production from the other production wells are resumed, while continuing to inject a portion of the produced formation hydrocarbon fluids into the converted production well.

Examples of the extraneous fluid include produced formation hydrocarbon fluids, which may be treated with thickeners to increase the viscosity thereof, butane and propane, all being miscible with the formation fluids.

The continuous injection into the converted well establishes a system of pressure gradients which on one side of the well are directed opposite to the pressure gradients associated with the driving fluid. A point of equilibrium of forces is established wherever the components of pressure gradient directed away from the converted well are equal and opposite to the components directed toward that well. The locus of all such equilibrium points establishes a stable interface, normally teardrop shaped, around the converted well. The shape and size of this interface will depend upon the interrelationship of many factors, primarily, geometry of well positions, relative permeabilities and viscosities, and well rate distributions. Control of any of these factors can be used thereby to enhance the effectiveness of the method. Thus with the offset well on production, the flow gradients are controlled so that the interface is spread out and the normal teardrop shape of the injected slug is changed toward the direction of the offset production well to result in a wider gradient barrier.

Since the injected driving fluid cannot penetrate this gradient barrier, it must travel a roundabout and longer flow path to reach the final production well, thereby delaying cusping into the end production well and allowinga longer period for the advance of the interface between the driving fluid and the formation fluids before breakthrough at this further well. When production at this end well is continued after breakthrough, the converted well is shut in and the continuing production results in recovery of the injected produced hydrocarbon fluids, along with remaining in place formation fluids.

Many variations of this basic procedures are possible, and some will be more advantageous than others for particular geometries of well positions, and reservoir and fluid parameters. But they will have certain things in common:

I. An intermediate well between and a well offset from the line through an injection source and a production well either exist or are added;

2. At breakthrough into this intermediate well or the offset well (or some time prior thereto), the injection is suspended and a volume of fluid, such as a portion of the produced fluid hydrocarbons, is injected into the well suffering breakthrough; this injection may be done with or without simultaneous production from other wells;

3. Injection of driving fluid is resumed at the principal injection well, and injection of the barrier fluid is continued into the well suffering breakthrough at a rate equal to a percentage of the production rates;

4. This continues, even though other existing wells may be "captured by injected driving fluid and closed in, until breakthrough of the injected fluid into the production well on the other side of the converted well;

.5. At this time, or in some cases even before it, the injection of barrier fluid is stopped; continued injection of driving fluid on one side and continued production on the other will move the carrier fluid completely into the production well if recovery of this fluid is desired (as, for example, if the barrier fluid used comprises produced fluid hydrocarbons).

Other objects, advantages and features of this invention will become apparent from a consideration of the specification with reference to the figures of the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 discloses four units of an inverted five-spot pattern (prior art);

FIG. 1a is illustrative of the interface advance in the form of a cusp toward a corner production well in one quadrant of such a five-spot undergoing secondary recovery (prior art);

FIG. 2 illustrates the movement of the interface during phases of a production in one quadrant of a 13-well quadrilateral side well pattern undergoing secondary recovery (prior art);

FIG. 3 discloses one quadrant of a nine-well diagonal pattern, l u i g t rctr atq the ."P.?l. 1 i9 1 l9 fi!l between the injection well and the corner production well resulting from injection of a small volume of produced hydrocarbon fluids into a control well, with all other wells temporarily shut in;

FIGS. 4a and 4b and FIGS. 5a and 5b illustrate the effects of an offset production well on a line drive of at least three wells wherein a dynamic barrier is imposed on the in-line production well suffering breakthrough;

FIGS. 6a and 6b illustrate an alternate showing of the effect of an offset production well in a 5 well grouping spreading the cusp interface and with dynamic barriers imposed in turn on it and the in-line production well suffering breakthrough;

FIGS. 7a, 7b, 7c and 7d illustrate the application of the principle of FIG. 6a to a uniformly spaced nine-well grouping, (including a seven-well grouping), starting at one end of a diagonal axis thereof; and

FIGS. 8a and 8b illustrate the changes in well functions in accordance with the movement of the spread interface during the several phases of the production program in a 13-well grouping undergoing secondary recovery.

The objects of the invention are achieved by the use of a combination of production wells used as control wells to modify the interface or delay the breakthrough of injected drlving fluid into the outcrttiost production wells, typically in pattern units, by reshaping and/or retarding the development of the usual cusp interface between the formation and injected fluids.

The specification and the figures of the drawings schematically disclose and illustrate the practice and the advantages of the invention with well patterns and areal sweep examples which are obtainable and have been observed both in secondary recovery operations and in potentiometric model studies which, simulate secondary recovery operatiOns'The model studies indicate a sweep obtained in an ideal reservoir, although the recovery from an actual sweep of a particular field may be greater or less, depending on field parameters.

Throughout the figures of the drawings, the same symbols will be maintained as follows: P P, and P, represent, respectively, production wells at the comers,along the sides, and the interior control wells of a pattern or well arrangement; the left diagonal shading represents the reinjected produced hydrocarbon fluids; and, an open circle indicates a well site, a solid circle indicates a production well, a crossed circle indicates a shut-in well, an upwardly arrowed open circle indicates an injection well, and a downwardly arrowed solid circle, a converted well. The diagonal xx in FIGS. 7a to 7d inclusive and 8a and 8b represents an axis of an injection well and a pair of in-line production wells.

Referring to FIG. 1, there is disclosed four units of an inverted five-spot pattern wherein the corner wells of each pattern unit are production wells, while the inner central well is used for injection.

FIG. 1a illustrates the growth of the cusp in one quadrant of an inverted five spot pattern unit, wherein the secondary flooding fluid is injected into the central well and production is maintained at the corner wells until breakthrough, to result in a sweep of approximately 71 percent.

FIG. 2 illustrates the development of the cusp toward the comer production well, retarded by spreading of the gradients, production being maintained at both the corner and side production wells until breakthrough at the side wells, at which time they are shut in for the end of the first phase, and then continuing production at the corner wells only, until breakthrough, for the end of the second phase.

In FIG. 3, the invention illustrates the improvement provided by one embodiment of this invention over the disclosures of the prior art in FIGS. 1, la, and 2. With injection of the driving fluid via the central well of a nine-well diagonal grouping production is maintained via the interior control well, P,, (or also via the corner production well, P until breakthrough thereat, to form the interface indicated at A,P,A I

. maintained till breakthrough at the corner well, P the sweep (not indicated) would increase by 9 percent, for a total sweep of 66 percent.

Instead, a volume of produced formation hydrocarbon fluids equal to a predetermined value, e.g. about 15 percent of a pattern unit volume, is injected through the interior control well, P,, from which production has ceased, and the cusp driven back by the resulting injected bubble of hydrocarbon fluids, as shown in section with left diagonals at C FIG. 3, and the interface distorted as indicated by the dash outline at A A the point of the cusp being driven back from the control well, P,, while the flanks advance from the line A,P,A,. The changes in the shapes of the sides of the interface haVe been exaggerated for purposes of clarity.

When production is initiated (or resumed) at the corner well, P and with about 50 percent of produced hydrocarbon fluids being injected continuously, into the formation via well P the cycling hydrocarbon fluids form a stable teardrop bubble, as indicated in left diagonal sectioning at C,, FIG. 3. The envelope of this bubble represents the surface of equilibrium of pressure gradient forces.

By the time the driving fluid gets around the gradient barrier to achieve breakthrough at the corner production well, P the sweep of the formation fluids has been increased 31 percent, as indicated by the interface at B P B for a total sweep of 88 percent.

Finally, if production at P, is continued after breakthrough with the control well P, closed in, maximum gradients are reestablished along the axis of the wells, and virtually all the injected produced fluid hydrocarbon fluids can be recovered quickly along with additional formation fluids miscible therewith, bringing total sweep to over 90 percent before an appreciable percentage of injected driving fluid is produced.

The sweep can be increased either by forming a larger initial bubble or using a greater fraction of the produced hydrocarbon fluids for injection back into the formation via the converted control well.

FIG. 4a illustrates two phase of an in-line production drive wherein a dynamic barrier of produced hydrocarbon fluids is injected into the formation via the second well in line, 2, at which breakthrough of the driving fluid, injected via the first well in line 1, has occurred. During the second phase with the reinjection via well 2, the third well in line 3, is placed on production and the fourth well, 4, is offset and may be on a standby basis, i.e. shut in, for the sweep discussed, or may be put on production. Thus, the start of the third phase is illustrated in FIG. 4a, with injection at 1, with sweep indicated by right diagonals, reinjection at 2 (left diagonals) and production at 3 and 4 until breakthrough of the latter as indicated by the dash outline in FIG. 4b, showing the spreading of the cusp interface. The end of the fourth phase concludes at breakthrough at 3, leaving-only a lens of produced hydrocarbon fluids, wells 2 and 4 having been shut in and injection being continued via well 1.

In FIG. 5a, production via wells 2 and 4 (and well 3 if so programmed) continues till breakthrough of the driving fluid injected via well I, at which time, a slug of produced hydrocarbon fluids is injected back into the formation via the well suffering breakthrough, which may be either the production well in line, 2, as illustrated, or the offset well 4, to which the cusp has spread. Thereupon, a dynamic barrier of produced hydrocarbon fluids is imposed on well 2 and production is either initiated at well 3 or resumed at wells 3 and 4, with injection via well I until breakthrough at well 4, the start of this phase being illustrated in FIG. 5a.

In FIG. 5b, the ends of the third and fourth phases are illustrated with breakthrough at the third well in line, 3, wells 2 and 4 being shut in and injection being maintained via well 1. The dash outline through 4 indicates how much the cusp has spread.

In FIG. 6a, there is illustrated the imposition of dynamic barriers on wells 2 and 4 upon breakthrough of driving fluid thereat, with injection via well 1 and production from well 3, and in FIG. 6b, the conclusion of production via well 3 is illustrated, injection being continued via well 1, wells 2 and 4 being shut in.

FIGS. 7a to 7d inclusive illustrate how the disclosed invention may be applied to a uniformly spaced nine-well grouping, the production operation being initiated along diagonal of wells 1, 2 and 3, continued from offset side wells 4 and 4, thence along the other diagonal through wells 6, 2 and 6', then the other offset side wells 5 and 5' to conclusion at the remaining corner well 3.

Upon breakthrough of the driving fluid injected via well I at the side wells 4 and 4' dynamic barriers are imposed thereupon and production continued from well 2 till breakthrough thereat, as illustrated in FIG. 7a, the cusp interface having been spread by the offset side well production.

In FIG. 7b, production is continued via the corner wells 6 and 6 with further cusp spreading and with dynamic barriers imposed on wells 4, 2 and 4 to spread the interface between the driving and driven fluids.

Upon breakthrough at the latter production wells, dynamic barriers are imposed thereupon and production is continued till breakthrough via the other offset side wells 5 and 5 and the remaining corner well v3, as indicated in FIG. 70. Thereupon, dynamic barriers are imposed on them and the production is concluded upon breakthrough at the corner well 3, as illustrated in FIG. 7d.

FIGS. 8a and 8b disclose the basic nine-spot pattern modified by the addition of four interior control wells, which can be positioned along the diagonals of the pattern for best advantage as indicated previously. It can be visualized also as a four-unit five-spot pattern, wherein the injection wells of the inverted five-spot pattern units have been converted to production wells, and the innermost production well of the four-unit five-spot pattern has been converted into an injection well. With such a conversion, the positions of the control wells have been predetermined and may not be situated for best efiect.

As illustrated in one quadrant of the pattern, the first phase of the production method requires injecting driving fluid via the central well and production initiated and maintained at the remaining 12 wells of the pattern until breakthrough is achieved at the four interior control wells, as shown by the dash outline in FIG. 8a. Then these interior production wells are converted to injection wells for receiving produced formation hydrocarbon fluids while production is continued from the comer wells P and the side wells P,, spreading the cusp until breakthrough thereat, as illustrated by the solid outline in FIG. 8a, the teardrop shape cross section indicating the returned hydrocarbon fluids. As indicated in FIG. 8b, the four side wells are converted to injection wells for produced hydrocarbon fluids, the bubble being shown in dash outline, and production is continued at the corner production wells P until breakthrough of the injection fluid occurs thereat. If production after breakthrough at the corner wells is continued, it becomes feasible to recover the teardrop bubbles for additional sweep, leaving slivers of unswept areas adjacent thereat.

The effect of continuous injection of a control well fluid into the formation between the well injecting driving fluid and a production well is tocreate an interior envelope of cycling fluid between the control well and the production well which the driving fluid cannot penetrate. This condition forces the driving fluid to move around the barrier thus retarding cusp formation toward the production well, giving longer time for sweep before interface breakthrough. Clearly, the wider the barrier can be made, the better will be the sweep.

Factors which will cause such a barrier to be wide are:

A. higher viscosity of the control well fluid than of the driving fluid;

B. high control well injection rates as a percentage of production;

C. production from side wells during the control well injection phase;

D. dual control wells straddling the axis through the injection and production wells to form a wider bubble, depending on the spacing between the straddle wells.

Any pattern and/or rate distribution which retards the development, or the advance, of a cusp towards production wells will increase the sweep of a field. Two principal means of doing this have been cited above, viz (a) pinning" down the cusp by locating production wells between the injection source and the outer production wells, and keeping such inner (or control) wells on production after breakthrough; and (b) spreading out the cusp by pulling the front toward side wells until breakthrough thereat before allowing the interface to proceed toward the corner production wells of a pattern unit.

I-Ierein has been disclosed another method of delaying the advance of the interface in the form of a cusp toward an outer production well by locating a dynamic barrier of produced fluid hydrocarbons between an injection well and the outer production well in combination with a spreading of the cusp by the use ofoffset side wells in an in line production drive.

Although emphasis has been placed in this disclosure on the practice of this invention as directed to a secondary recovery operation, particularly employing water or other similar aqueous fluid as the injection displacement fluid, the advantages obtainable in the practice of this invention are also realized in primary hydrocarbon production operations wherein the hydrocarbon-bearing formation is under the influence of either a water or gas drive, or both a water and gas drive, and also in the instance of a secondary recovery operation wherein a gas, such as natural gas, is employed as the injection fluid. Moreover, the invention is applicable particularly to an arrangement of a pair of production wells in line with an injection well under the influence of an active water drive.

lclaim:

1. A method of producing formation fluids including hydrocarbons from an underground hydrocarbon-bearing formation which comprises penetrating said formation with at least three wells substantially in line, a first well, a second well and a third well, the second and third wells being on one side of said first well with said second well closer thereto, and a well offset from the line of wells and adjacent said second well, injecting an extraneous fluid into said formation via said first well to displace fluids including hydrocarbons in said formation toward said second and third wells and the offset well, producing said formation fluids including hydrocarbons from said formation via the second and offset wells, recovering formation hydrocarbon fluids from produced formation fluids, ceasing producing said formation fluids via the production well suffering breakthrough of said extraneous fluid thereat and thereupon injecting a portion of said formation hydrocarbon fluids into said formation via said production well suffering breakthrough while injecting extraneous fluid via said first well, and thereafter producing formation fluids via said third well and the other original production well.

2. In a method as defined in claim 1, maintaining producing said formation fluids including hydrocarbons from said formation via said third well and said others original production well while injecting formation hydrocarbon fluids into said formation via said production well suffering breakthrough.

3. in a method as defined in claim 1, the production of formation fluids via said second and offset wells being simultaneous.

4. In a method as defined in claim 1, converting said second and offset wells to injection wells for produced hydrocarbon fluids as each suffers breakthrough of said extraneous fluid injected via said first well, meanwhile producing formation fluids via said third well, said offset well being one of a pair which straddles the line of three wells.

5. In a method as defined in claim 1, continuing injecting produced formation hydrocarbon fluids via said well suffering breakthrough for a predetermined period while maintaining producing via the remaining production wells.

6. in a method as defined in claim 1, said injecting of hydrocarbon fluids via said production well suffering breakthrough being of a predetermined percentage of the pattern unit volume in the amount of about percent, said extraneous fluid being selected from the group consisting of butane, propane and produced hydrocarbon fluids.

7. in a method of producing formationfiuids as defined in claim 1, said three wells in line being part of a five well grouping, said offsetwell being one of a pair which straddles the line of said three wells.

8. In a method of producing formation fluids as defined in claim 1, said three wells in line being part of a seven-well geometric grouping.

9. In a method of producing formation fluids as defined in claim 1, said three wells in line being part of a uniformly spaced nine-well grouping.

10. In a method as defined in claim 9, said second well being spaced away from said first well by at least three quarters of the distance between said first well and said third well.

11. In a method of producing formation fluids as defined in claim 1, said three wells in line being part of a 13-well grouping, wherein the central well of said grouping is an injection well and the remaining grouping wells are production wells arranged equally along the sides and on the diagonals of a quadrilateral, said second and third wells being arranged along a diagonal thereof.

12. In a method of producing formation fluids as defined in claim 11, simultaneously initiating producing said formation fluids via all of said production wells.

13. A method of producing formation fluids including hydrocarbons from an underground hydrocarbon-bearing formation under the influence of an active aquifer which comprises penetrating said formation with a pair of production wells in line with the direction of advance of said aquifer, and

a third production well offset from said line of production wells and adjacent one of said pair of production wells, producing formation fluids including hydrocarbons displaced by said aquifer via said pairs of production wells and the offset well until breakthrough of the interface between the formation fluids and said aquifer at a production well, thereupon ceasing producing formation fluids thereat and injecting thereinto an extraneous fluid to provide a dynamic gradient barrier at the interface where said breakthrough has occurred, meanwhile producing formation fluids including hydrocarbons from said formation via the remaining production wells while maintaining said gradient barrier.

14. In a method of producing formation fluids including hydrocarbons as defined in claim 13, said extraneous fluid being selected from the group consisting of butane, propane and produced hydrocarbon fluids and being of predetermined volume, starting with a percentage of the pattern unit volume and continuing with a percentage of the produced of formation fluids from another production well.

15. In a method of producing formation fluids including hydrocarbons as defined in claim 14, the predetermined volume amounting to 15 percent of the pattern unit volume. 

2. In a method as defined in claim 1, maintaining producing said formation fluids including hydrocarbons from said formation via said third well and said others original production well while injecting formation hydrocarbon fluids into said formation via said production well suffering breakthrough.
 3. In a method as defined in claim 1, the production of formation fluids via said second and offset wells being simultaneous.
 4. In a method as defined in claim 1, converting said second and offset wells to injection wells for produced hydrocarbon fluids as each suffers breakthrough of said extraneous fluid injected via said first well, meanwhile producing formation fluids via said third well, said offset well being one of a pair which straddles the line of three wells.
 5. In a method as defined in claim 1, continuing injecting produced formation hydrocarbon fluids via said well suffering breakthrough for a predetermined period while maintaining producing via the remaining production wells.
 6. In a method as defined in claim 1, said injecting of hydrocarbon fluids via said production well suffering breakthrough being of a predetermined percentage of the pattern unit volume in the amount of about 15 percent, said extraneous fluid being selected from the group consisting of butane, propane and produced hydrocarbon fluids.
 7. In a method of producing formation fluids as defined in claim 1, said three wells in line being part of a five well grouping, said offset well being one of a pair which straddles the line of said three wells.
 8. In a method of producing formation fluids as defined in claim 1, said three wells in line being part of a seven-well geometric grouping.
 9. In a method of producing formation fluids as defined in claim 1, said three wells in line being part of a uniformly spaced nine-well grouping.
 10. In a method as defined in claim 9, said second well being spaced away from said first well by at least three quarters of the distance between said first well and said third well.
 11. In a method of producing formation fluids as defined in claim 1, said three wells in line being part of a 13-well grouping, wherein the central well of said grouping is an injection well and the remaining grouping wells are production wells arranged equally along the sides and on the diagonals of a quadrilateral, said second and third wells being arranged along a diagonal thereof.
 12. In a method of producing formation fluids as defined in claim 11, simultaneously initiating producing said formation fluids via all of said production wells.
 13. A method of producing formation fluids including hydrocarbons from an underground hydrocarbon-bearing formation under the influence of an active aquifer which comprises penetrating said formation with a pair of production wells in line with the direction of advance of said aquifer, and a third production well offset from said line of production wells and adjacent one of said pair of production wells, producing formation fluids including hydrocarbons displaced by said aquifer via said pairs of production wells and the offset well until breakthrough of the interface between the formation fluids and said aquifer at a production well, thereupon ceasing producing formation fluids thereat and injecting thereinto an extraneous fluid to provide a dynamic gradient barrier at the interface where said breakthrough has occurred, meanwhile producing formation fluids including hydrocarbons from said formation via the remaining production wells while maintaining said gradient barrier.
 14. In a method of producing formation fluids including hydrocarbons as defined in claim 13, said extraneous fluid being selected from the group consisting of butane, propane and produced hydrocarbon fluids and being of predetermined volume, starting with a percentage of the pattern unit volume and continuing with a percentage of the produced of formation fluids from another prOduction well.
 15. In a method of producing formation fluids including hydrocarbons as defined in claim 14, the predetermined volume amounting to 15 percent of the pattern unit volume. 